1. Field of the Invention
The present invention relates to a gene expression system in astrocyte cells. Such a system could be used to express gene products not normally present, or to increase the expression of endogenous genes in astrocytes and transgenic animals. It could also be used to curtail gene expression as, for example, by the use of antisense RNA. Among the many uses of the present gene expression system are the evaluation of the roles of various gene products in brain development and function, and the creation of transgenic animal models possessing gliomas and afflicted with Alzheimer's disease. Such an animal model would permit the evaluation of various chemical compounds for their therapeutic effect on Alzheimer's disease. Additionally, the model could be used to test the effect of various materials and foods on that brain disease.
2. Description of Related Art
Glia make up about half the cells in the human brain. Although once thought little more than a space-filling glue, these cells are now known to participate actively in brain physiology. There are two major types of glial cells: oligodendrocytes, which ensheath nerve fibers with myelin, and astrocytes, which perform a variety of structural and metabolic functions. The list of the functions performed by astrocytes has been growing steadily, and now includes processing neurotransmitters, controlling extracellular ion levels, regulating the direction and amount of nerve growth, maintaining the blood-brain barrier, and participating in immune reactions (reviewed in 1, 2). Understanding astrocyte function is therefore central to understanding brain function. To study transcriptional controls in astrocytes, the present inventors have focused on the gfa gene, which encodes glial fibrillary acidic protein (GFAP), an intermediate filament protein found abundantly, and almost exclusively, in astrocytes (reviewed in 3). Besides serving as a convenient astrocyte-specific marker, GFAP is of interest in its own right. Synthesis of the protein is developmentally regulated, the precursors of astrocytes initially synthesizing vimentin, and then switching to GFAP near the time of birth (4). In adults, levels of GFAP increase as a result of the proliferation of astrocytes (reactive gliosis) that occurs in response to a variety of physical, chemical, and etiologic insults, including Alzheimer's disease, epilepsy, and multiple sclerosis (1, 2). GFAP is a major component of the gliotic scars which result from gliosis, and which may interfere with subsequent reinnervation.
The present inventors have previously described isolation of human cDNA and genomic clones of gfa (5), and have analyzed the structure and function of its basal promoter (6, 7). Transgenic mice have been prepared by injecting a glial fibrillary acidic protein (GFAP) hybrid construct into the germ line of mice (36). As an initial step in developing an animal model for Alzheimer's disease, a 4.5 kb DNA fragment from the 5' end of the human APP gene, which mediates neuron-specific gene expression in the CNS of transgenic mice, has been identified (37). The cloning and sequencing of the 5' flanking region of the mouse GFAP gene and the astrocyte-specific expression of a GFAP gene construct in transfected glioblastoma cells, has been reported (38). How the expression of the GFAP gene is affected by cis-acting elements in human astrocytoma and rat glioma cells has been explored (39).
It has been shown that transgenic mice whose cells contain activated oncogenic sequences are useful as a model for evaluating potential carcinogens, as well as a model for testing materials thought to confer protection against the development of neoplasma (40).